Some electromagnetic flow meters of the related art are configured to extract an electromotive force generated in a fluid flowing in a measurement tube by using an electropotential detection electrode. The electropotential detection electrode, typically made of a stainless steel, is made of various materials depending on corrosiveness of detection targets. Examples of electrode materials having high corrosive resistance include precious metal materials such as platinum as disclosed, for example, in PTL 1 in many cases. The precious metal materials have such a disadvantage as having difficulty to achieve desired shapes depending on the shape due to their low strength. In order to solve the disadvantage described above, forming an electrode by using a non-precious metal material as a base metal and covering the electrode with a precious metal material is conceivable as described in PTL 1.
However, when an abrasive substance is included in the fluid flowing in the measurement tube, the precious metal material covering the electrode may be peeled off. In addition, the precious metal material covering the electrode may be peeled off due to an impact applied to the measurement tube, corrosion of the measurement tube, or a defect at the time of manufacture.
When the precious metal material is peeled off, an electrochemical noise is generated due to an electropotential difference between the base metal and the precious metal material. This noise corresponds to an output noise from the electromagnetic flow meter.
The problem of generation of the noise due to peeling off of the metal material that covers the electrode may be solved by making an electrode body with an insulating material, covering a surface of the insulating material with a precious metal material, and using the precious metal material as a conduction path as proposed by an applicant of the present application in PTL 2. Described in PTL 2 is an electropotential detection electrode 3 having such structure that a base material 1 made of a ceramic, which is an insulating material, is covered with a conductor 2 made of a metal having corrosive resistance as illustrated in FIG. 9. The electropotential detection electrode 3 includes a first small diameter portion 3a to be inserted into a hole 5 formed in a measurement tube 4 for inserting an electrode, a large diameter portion 3b located outside the measurement tube 4, and a second small diameter portion 3c projecting from the large diameter portion 3b in a direction opposite from the first small diameter portion 3a. A lead wire 6 is connected to the second small diameter portion 3c. 
The second small diameter portion 3c has a function as a terminal for connecting the lead wire 6 and a function as a grip that an operator holds with fingers for attaching and removing the electropotential detection electrode 3 with respect to the measurement tube 4. Therefore, the electropotential detection electrode 3 of the related art requires that a terminal portion 8 including the second small diameter portion 3c is coupled to a main body portion 7 (see FIG. 9) including the first small diameter portion 3a and the large diameter portion 3b. 
However, for configuring the electropotential detection electrode 3 like the electrode described in PTL 2, if a ceramic is employed as an insulating material of the base material 1, the terminal portion 8 and the main body portion 7 are preferably formed separately. It is because of such advantages that forming the main body portion 7 and the terminal portion 8 separately improves a strength of these members and reduces difficulty in manufacture, and a surface area to be applied with a material for conductor 2 is reduced. In order to form the main body portion 7 and the terminal portion 8 separately in this manner, a coupling structure for coupling the main body portion 7 and the terminal portion 8 is required.
Conceivable coupling structures include, for example, press-fitting, fastening with an insert screw, brazing, and adhesion. When the base material 1 is made of a brittle material, such as alumina, the coupling structure cannot be achieved with the insert screw or brazing because the press-fitting may cause breakage, and the adhesion cannot achieve electric conduction.
In order to insert the insert screw into the main body portion 7 of the electropotential detection electrode 3, configurations as illustrated in FIG. 10 and FIG. 11 are conceivable. In FIG. 10 and FIG. 11, the same or similar members as or to the members described with reference to FIG. 9 will be denoted by the same reference signs and detailed description will be omitted as appropriate.
An electropotential detection electrode 11 illustrated in FIG. 10 and FIG. 11 includes the main body portion 7 formed by covering the base material 1 made of a ceramic with a conductor and the terminal portion 8 attached to the main body portion 7 via a coupling structure 12. The coupling structure 12 includes an insert screw 13 including a female screw member embedded in the main body portion 7 and a male screw 14 of the terminal portion 8 screwed into the insert screw 13.
The insert screw 13 illustrated in FIG. 10 is provided inside the large diameter portion 3b of the main body portion 7. Therefore, the large diameter portion 3b is formed to have a thickness that can store the insert screw 13.
The insert screw 13 illustrated in FIG. 11 is disposed through the large diameter portion 3b and a distal end portion enters into the first small diameter portion 3a. Therefore, the large diameter portion 3b is formed to have a smaller thickness compared with the large diameter portion 3b illustrated in FIG. 10, and the first small diameter portion 3a is formed to have a relatively larger outer diameter.